Any rope will do...doesn't have to be any special MARINE GRADE rope...last time I bought some for the rub rail was from a hardware store and be sure to get the right size diameter for YOUR rub rail...I wouldn't get a cotton fiber rope...needs to be nylon or some synthetic fiber...color is your option...
When I replaced mine a few years ago I used 5/8" three strand. I greased up the rubber part with dawn dish soap and spread the flanges with the knobbed end of a craftsman screwdriver handle as I went along inserting the rope. Be sure to leave it "long" for a while hanging in the splash well as it will shrink up after some time. Afterwards tape up the ends and cut off the excess.